"That weapon defeats foes with one hit. However, the reverse is also true, as its wielder will also fall to a single strike. You can use this weapon's ability only twice within a set period of time...only when it is glowing. When enough time passes, the weapon will regain its sheen along with its power."
― Maz Koshia describing the One Hit Obliterator (The Legend of Zelda: Breath of the Wild)
The user can kill the target with a single strike. The attack carries special attributes, such as lethal poison or death inducement, that would result in an instant death the moment it connects, disregarding on the location of contact or the target's remaining vitality.
Other, less sophisticated methods are either the attack strikes the target's vital point, or it just carries devastating firepower and brute force to outright kill the opponent.
The instant kill is capable of bypassing regeneration of anylevel, as it deals death before the opponent has a chance to rejuvenate.
Akame's (Akame Ga Kill!) One Cut Killer: Murasame kills any it cuts with a cursed poison, even the user if not careful while cleaning the blade.
Byakuya Kuchiki (Bleach)
White Bomber (Bomberman 64: The Second Attack) detonating a Gravity Bomb, which will kill any and all it sucks in instantly, including White Bomber if he's not careful.
Omnimon X (Digimon) is able to one hit kill opponents with his All Delete attack.
Death Note (Death Note)
Agatsuma Zenitsu (Demon Slayer: Kimetsu no Yaiba)
Peter Pettigrew (Harry Potter) uses the Killing Curse, reputably unstoppable and irreversible, to instantly kill Cedric Diggory.
Kaguya Ōtsutsuki (Naruto) using her Murderous Bone Ash/All-Killing Ash Bones that will cause her victims to disintegrate into ashes the moment it pierces the body.
Magellan's (One Piece) Kinjite poison spreads like disease through organic material and rocks, and kills instantly the moment it infects the target.
Link's (The Legend of Zelda: The Wind Waker) Light Arrows will instantly kill any evil forces it pierces, bypassing any defenses and vanquishing it in pure light.
Gae Bolg (TYPE-MOON) divides into countless thorns that destroy target's innards when it pierces the heart.
Certain weapons the Inklings (Splatoon) use deal enough damage in one hit to "splat" their enemies instantly.
Agent 3 (Splatoon)
Certain weapons the Octarians (Splatoon), such as DJ Octavio, use deal enough damage in one hit to "splat" Agent 3 instantly.
The Vorpal Sword (Jabberwock) can kill the Jabberwocky with one hit. In other lore, it can kill anything with a single cut.
Alan’s (Marchen Awakens Romance) superhuman strength lets him even destroy enormous monsters with a single punch.
Machina's (Dies Irae) Briah, Midgardr Völsunga Saga, allows him, with merely a single blow, to erase the existence of anything with a history, including living beings, inanimate objects and even concepts.
Rose's Sword (Steven Universe) was made to be able to destroy a gem's physical form with a single swipe.
The One-Hit Obliterator (The Legend of Zelda: Breath of the Wild) is a divine weapon capable of felling a foe in a single strike at the expense of exhibiting the same circumstances to the wielder.
Yui (Sword Art Online) wielding the Object Eraser, a flaming blade which can destroy anything it's used against, such as a boss monster that would be impossible to beat, in a single strike.
Kenshiro (Fist of the North Star)
Kyou Kai (Kingdom)
Rin Ko of the Four Heavenly Kings (Kingdom)
Kou Yoku of the Thunder (Kingdom) strikes hard and fast like his nickname.
Gyou' Un of Zhao (Kingdom) kills a group of Qin soldiers with one swing of his Podao.
Mehrunes' Razor (Elder Scrolls) is the Daedric Artifact of Mehrunes Dagon, and has a chance of killing any being with a single cut.
Jirou Sorashima (Wallman)
Using the Hadoken, Mega Man X (Mega Man X series) can destroy most enemies and bosses with a single hit.